HTML4 and CSS2 currently support media-dependent style sheets tailored for
different media types. For example, a document may use sans-serif
fonts when displayed on a screen and serif fonts when printed. "Screen" and
"print" are two of media types that have been defined. Media Queries
extend the functionality of media types by allowing more precise labeling of
style sheets.
A Media Query consists of a media type and one or more expressions to
limit the scope of style sheets. Among the media features that can
be used in media queries are "width", "height", and "color". By using Media
Queries, presentations can be tailored to a specific range of output devices
without changing the content itself.
Status of this document
This section describes the status of this document at the time of its
publication. Other documents may supersede this document. The latest status
of this document series is maintained at the W3C.
This document is produced by the CSS Working Group (part of the Style Activity, see summary)
and is one of the modules of the upcoming CSS3 specification.
The specification is being put forth as a Candidate
Recommendation, which means that W3C believes this specification is
stable. It can still be updated by the Working Group, but only to clarify its
meaning. If, unexpectedly, serious problems are found, it will be returned to
Working Draft status. Otherwise, as soon as the exit criteria are met, it
will progress to become Proposed Recommendation. In either case, the duration
of the Candidate Recommendation phase will be at least six months (July 2002
- Jan 2003).
For this specification to exit the CR stage, the following conditions
shall be met:
There must be at least two interoperable implementations. For the purposes
of this criterion, we define the following terms:
interoperable
passing the respective test case(s) in the CSS test suite, or, if the
implementation is not a web browser, an equivalent test. Every relevant test
in the test suite should have an equivalent test created if such a UA is to
be used to claim interoperability. In addition if such a UA is to be used to
claim interoperability, then there must one or more additional UAs which can
also pass those equivalent tests in the same way for the purpose of
interoperability. The equivalent tests must be made publicly available for
the purposes of peer review.
implementation
a user agent which:
implements the specification.
is available (i.e. publicly downloadable or available through some other
public point of sale mechanism). This is the "show me" requirement.
is shipping (i.e. development, private or unofficial versions are
insufficient).
is not experimental (i.e. is intended for a wide audience and could be
used on a daily basis.)
A minimum of sixth months of the CR period must have elapsed. This is to
ensure that enough time is given for any remaining major errors to be caught.
The specified technology must not be harmful for accessibility.
HTML4 [HTML401] and CSS2 [CSS2] currently support media-dependent
style sheets tailored for different media types. For example, a document may
use different style sheets for screen and print. In HTML4, this can be
written as:
Inside a CSS style sheet, one can declare that sections apply to certain
media types:
@media screen {
* { font-family: sans-serif }
}
The "print" and "screen" media types are defined in HTML4. The complete
list of media types in HTML4 is: "aural", "braille", "handheld", "print",
"projection", "screen", "tty", "tv". CSS2 defines the same list with the
addition of "embossed" to differentiate between braille tactile feedback
devices and braille printers. Also, "all" is used to indicate that the style
sheet applies to all media types.
Media-specific style sheets are supported by several user agents. The most
commonly used feature is to distinguish between "screen" and "print", and
"projection" is beginning to catch on.
There have been requests for ways to describe in more detail what type of
output devices a style sheet applies to. Fortunately HTML4 foresaw these
requests and defined a forward-compatible syntax for media types. Here is a
quote from HTML4,
section 6.13:
Future versions of HTML may introduce new values and may allow
parameterized values. To facilitate the introduction of these extensions,
conforming user agents must be able to parse the media attribute value as follows:
The value is a comma-separated list of entries. For example,
media="screen, 3d-glasses, print and resolution > 90dpi"
is mapped to:
"screen"
"3d-glasses"
"print and resolution > 90dpi"
Each entry is truncated just before the first character that isn't a US
ASCII letter [a-zA-Z] (Unicode decimal 65-90, 97-122), digit [0-9] (Unicode
hex 30-39), or hyphen (45). In the example, this gives:
"screen"
"3d-glasses"
"print"
The hypothetical example from the HTML4 specification is worth discussing
in more detail:
There are two logical operators in the example above. The comma denotes
the logical OR, and the word "and" denotes the logical AND. The logical OR is
already part of the HTML4 specification. The logical AND operator, however,
is not defined.
Another operator used in the example above is "greater-than", denoted by
">". The square brackets and the ampersand character are reserved in HTML
and XML and are therefore not suitable for use.
Finally, two different vocabularies are used: one for the media features
("resolution" is used above) and one for units (e.g. "dpi").
2. Requirements
[informative]
In order for a solution to provide useful new functionality while
remaining compatible current with user agents supporting HTML4, the following
requirements should be fulfilled:
The syntax must fit into the media type syntax reserved in. HTML4
The solution must describe, in some detail, what kind of devices a style
sheet can be applied to.
The syntax must be usable in future versions of CSS and XHTML.
The solution must offer more expressive power than the current solution
without adding significant extra costs.
The solution must use a vocabulary in line with CSS and HTML.
The syntax must leave room for future extensions.
3. Related work
[informative]
3.1. CC/PP
Within W3C the work on CC/PP (Composite Capabilities/Preference Profiles)
has addressed similar issues. The goal of the CC/PP framework is to specify
how client devices express their capabilities and preferences to a server.
Media Queries go the other way; it's the document that declares what kind of
media types a style sheet is suitable for. Still, the definition of media
features could be shared between CC/PP and Media Queries.
an indication of the color capabilities of the device
The last three media features are borrowed from RFC2534 [RFC2534].
It is noteworthy that the first two media features use a different naming
style (the so-called "CamelCase") than the last three.
3.2. RFC2534
Like CC/PP, RFC2534 also provides a framework for user agents to declare
their capabilities. In addition to "color", "pix-x" and "pix-y" (described
above) RFC2534 defines several other generic media features:
For several reasons, these are not directly reusable:
The media features described in RFC2534 has three types of values:
integers, rational numbers, and keywords. The CSS syntax also uses keywords,
but favors the combination of number and unit (e.g. "12px") for values
involving numbers. Not being able to use units would restrict the expressive
power of Media Queries.
The "ua-media" feature seems to be at the same level of abstraction as
the list of media types in HTML4/CSS2. One value ("screen") is found in both
categories, and RFC2534's "screen-paged" seems to be the same as "projection"
in HTML4/CSS2. The rest of the values in RFC2534 ("stationery",
"transparency", "envelope", "envelope-plain", "continuous") describe printed
output and may be candidates for new media types in HTML4/CSS2 if the list is
extended in the future.
The CSS WG has considered using names for page sizes in the past but
decided against doing so since there are a large number of standards
worldwide.
3.3. RFC2533
RFC2533 [RFC2533] describes a
syntax that can be used to define "feature sets" which are formed from
combinations and relations involving individual media features. Here is a
sample feature set:
(& (pix-x>=750) (pix-y<=500) (color=15) )
Alas, Media Queries could not use this syntax since '<', '>' and
'&' characters are, in their unescaped form, illegal inside HTML
attributes. In their escaped form ('>', '<' and '&')
they look ugly.
However, the same expressions can be written in Media Queries using
comma-separated lists (to express logical OR), the 'and' keyword (to express
logical AND) and 'min'/'max' prefixes to express '>=' and '<='.
4. Media Queries
[normative]
A Media Query consists of a media type and one or more
expressions involving media features. Here is a simple
example written in HTML:
<link rel="stylesheet" media="screen and (color)"
href="http://www.example.com/color" />
The example above expresses that a certain style sheet (found from
http://www.example.com/color) applies to devices of a certain media
type ("screen") with certain characteristics (it must be a color screen).
The Media Query is a logical expression that is either true or false. A
Media Query is true if the media type of the Media Query matches the media
type of the device where the user agent is running, and all expressions in
the Media Query are true. (The user agent is responsible for knowing which
one of the defined media types which most closely resembles the device.) In
the example above there is one expression ("(color)").
Several Media Queries can be combined in a comma-separated list. If one or
more of the Media Queries in the comma-separated list is true, the associated
style sheet is applied, otherwise the associated style sheet is ignored.
Consider this example written in CSS:
@media screen and (color),
projection and (color) { ... }
The comma expresses a logical OR, while the "and" keyword expresses a
logical AND. There is also a logical NOT in the syntax:
<link rel="stylesheet" media="not screen and (color)"
href="http://www.example.com/color" />
The presence of the keyword "not" at the beginning of the query will
negate the result. I.e., if the Media Query had been true without the "not"
keyword it will become false, and vice versa. User agents that only support
media types (as described in HTML4) will not recognize the "not" keyword and
the associated style sheet is therefore not applied.
The keyword "only" can also be used to hide style sheets from older user
agents:
<link rel="stylesheet" media="only screen and (color)"
href="http://www.example.com/color" />
User agents supporting this specification will process media queries
starting with "only" as if the "only" keyword was not present.
All Media Queries can be used in HTML, XHTML, XML [XMLSTYLE], and without changing their syntax. Here is
the same example written in all three languages:
<?xml-stylesheet media="screen and (color), projection and (color)"
rel="stylesheet" href="http://www.example.com/color" ?><link rel="stylesheet" media="screen and (color), projection and (color)"
rel="stylesheet" href="http://www.example.com/color" />@media screen and (color), projection and (color)
{ @import url(http://www.example.com/color) }
If a media feature does not apply to the device where the UA is running,
expressions involving the media feature will be false. For example, the media
feature "device-aspect-ratio" only applies to visual devices. On an aural
device, expressions involving "device-aspect-ratio" will therefore always be
false. Here is another example of a Media Query which is always false:
<link rel="stylesheet" media="aural and (min-device-width: 800px)"
href="http://www.example.com/color" />
Expressions will always be false if the unit of measurement does not apply
to the device. For example, the px unit does not apply to tty devices so the
following Media Query is always false:
<link rel="stylesheet" media="tty and (min-device-width: 800px)"
href="http://www.example.com/color" />
Note that the Media Queries in the two above examples would have been true
if the keyword "not" had been added to the beginning of the Media Query.
To avoid circular dependencies, it is never necessary to apply the style
sheet in order to evaluate expressions. For example, the aspect ratio of a
printed document may be influenced by a style sheet, but expressions
involving "device-aspect-ratio" will be based on the default aspect ratio of
the user agent.
CSS style sheets are generally case-insensitive, and this is also the case
for Media Queries.
The parentheses ("()") around expressions are required. A user agent
conforming to this specification must not process expressions without the
parentheses. By following this requirement, there will be room for future
extensions in the syntax.
Expressions involving unknown media features are always false. Consider
this example:
<link rel="stylesheet" media="all and (max-weight: 3kg)
and (color)" href="http://www.example.com/color" />
Since "max-weight" is an unknown media feature, the Media Query is false
and the associated style sheet will not be applied.
Syntactically, media features resemble CSS properties: they have names and
accept certain values. There are, however, several important differences
between properties and media features:
Properties are used in declarations to give information about
how to present a document. Media features are used in expressions to
describe requirements of the output device.
Most media features accept optional "min-" or "max-" prefixes to express
"greater or equal to" and "smaller or equal to" constraints. This syntax is
used to avoid "<" and ">" characters which conflict with HTML and XML.
Those media features that accept prefixes will most often be used with
prefixes, but can also be used alone.
Properties always require a value to form a declaration. Media features,
on the other hand, can also be used without a value in which case the
expression will be true if the media feature's value in the output device is
different from zero. For example, the "color" media feature can form
expressions without a value ("(color)"), or with a value
("(min-color: 1)".
This specification defines media features usable with visual and tactile
devices. Similarly, media features can be defined for aural media types.
6.1. width
Value:
<length> Applies to: visual and tactile media types Accepts min/max prefixes: yes
The "width" media feature describes the width of the rendering surface of
the output device. For continuous media, this is the width of the viewport
(as described by CSS2). For paged media, this is the width of the page box
(as described by CSS2).
For example, this Media Query expresses that the style sheet is usable on
printed output wider than 25cm:
<link rel="stylesheet" media="print and (min-width: 25cm)" href="http://...." />
This Media Query expresses that the style sheet is usable on devices with
viewport (the part of the screen/paper where the document is rendered) widths
between 400 and 700 pixels:
@media screen and (min-width: 400px) and (max-width: 700px) { ... }
This Media Query expresses that style sheet is usable on screen and
handheld devices if the width of the viewport is greater than 20em.
@media handheld and (min-width: 20em),
screen and (min-width: 20em) { ... }
The "em" value is based on the font size of the root element.
6.2. height
Value:
<length> Applies to: visual and tactile media types Accepts min/max prefixes: yes
The "height" media feature describes the height of the rendering surface
of the output device. For continuous media, this is the height of the
viewport (as described by CSS2). For paged media, this is the width of the
page box (as described by CSS2).
6.3. device-width
Value:
<length> Applies to: visual and tactile media types Accepts min/max prefixes: yes
The "device-width" media feature describes the width of the output device.
@media screen and (device-width: 800px) { ... }
In the example above, the style sheet will apply only to screens that
currently displays exactly 800 horizontal pixels. The "px" unit is of the
logical kind, as described in the Units section.
6.4. device-height
Value:
<length> Applies to: visual and tactile media types Accepts min/max prefixes: yes
The "device-height" media feature describes the height of the output
device.
<link rel="stylesheet" media="screen and (device-height: 600px)" />
In the example above, the style sheet will apply only to screens that
currently displays exactly 600 vertical pixels. The "px" unit is of the
logical kind, as described in the Units section.
6.5.
device-aspect-ratio
Value: <integer>
/ <integer> Applies to: bitmap media types Accepts min/max prefixes: yes
The "device-aspect-ratio" media feature describes the aspect ratio of the
output device. The value consists of two positive integers separated by a
"/".
In this specification, aspect ratio is defined as the number of horizontal
pixels over the number of vertical pixels.
For example, if a screen device has 1280 horizontal pixels and 720
vertical pixels (commonly referred to as "16:9"), the following Media Queries
will all match the device:
@media screen and (device-aspect-ratio: 16/9) { ... }
@media screen and (device-aspect-ratio: 32/18) { ... }
@media screen and (device-aspect-ratio: 1280/720) { ... }
@media screen and (device-aspect-ratio: 2560/1440) { ... }
Note that determining if a specified aspect ratio matches the device
aspect ratio can be done through multiplication (e.g., 1280*9 = 11520 =
720*16) to avoid floating point operations.
Using prefixes, portrait and landscape aspect ratios can
be expressed:
@media screen and (max-device-ratio: 1/1) /* portrait or square */
@media screen and (min-device-ratio: 1/1) /* landscape or square */
6.6. color
Value:
<integer> Applies to: visual media types Accept min/max prefixes: yes
The "color" media feature describes the number of bits per color component
of the output device. If the device is not a color device, the value will be
0. For example, these two Media Queries express that a style sheet applies to
all color devices:
@media all and (color) { ... }
@media all and (min-color: 1) { ... }
This Media Query expresses that a style sheet applies to color devices
with 2 or more bits per color component:
@media all and (min-color: 2) { ... }
For instance, if an 8-bit color system represents the red component with 3
bits, the green component with 3 bits and the blue component with 2 bits, the
"color" media feature will have a value of 2.
In a device with indexed colors, the "color" media feature will hold the
minimum number of bits per color component in the lookup table.
The described functionality is only able to describe color capabilities at
a superficial level. If further functionality is required, RFC2531 [RFC2531]
provides more specific media features which may be supported at a later
stage.
6.7. color-index
Value:
<integer> Applies to: visual media types Accepts min/max prefixes: yes
The "color-index" media feature describes the number of entries in the
color lookup table of the output device. For example, here are two ways to
express that a style sheet applies to all color index devices:
@media all and (color-index) { ... }
@media all and (min-color-index: 1) { ... }
This Media Query expresses that a style sheet applies to a color index
device with 256 or more entries:
<?xml-stylesheet media="all and (min-color-index: 256)"
href="http://www.example.com/..." ?>
6.8. monochrome
Value:
<integer> Applies to: visual media types Accepts prefixes: yes
The "monochrome" media feature describes the number of bits per pixel in a
monochrome frame buffer. If the device is not a monochrome device, the output
device value will be 0. For example, here are two ways to express that a
style sheet applies to all monochrome devices:
@media all and (monochrome) { ... }
@media all and (min-monochrome: 1) { ... }
For example, express that a style sheet applies to monochrome devices with
more than 2 bits per pixels:
@media all and (min-monochrome: 2) { ... }
Express that there is one style sheet for color pages and another for
monochrome:
<link rel="stylesheet" media="print and (color)"
href="http://..." /><link rel="stylesheet" media="print and (monochrome)"
href="http://..." />
6.9. resolution
Value:
<resolution> Applies to: bitmap media types Accepts min/max prefixes: yes
The "resolution" media feature describes the resolution of the output
device, i.e. the density of the pixels.
For example, this Media Query expresses that a style sheet is usable on
devices with resolution greater than 300 dots per inch:
@media print and (min-resolution: 300dpi) { ... }
6.10. scan
Value: progressive |
interlace Applies to: tv media types Accepts min/max prefixes: no
The "scan" media feature describes the scanning process of tv output
devices.
For example, this Media Query expresses that a style sheet is usable on tv
devices with progressive scanning:
@media tv and (scan: progressive) { ... }
6.11. grid
Value:
<integer> Applies to: all media types Accepts min/max prefixes: no
The "grid" media feature is used to query whether the output device is
grid or bitmap. If the output device is grid-based (e.g., a tty terminal, or
a phone display with only one fixed font), the value will be 1. Otherwise,
the value will be 0.
The "em" unit has a special meaning in grid output devices. Since the
exact with of an "em" cannot be determined, one "em" is assumed to be the
width of one grid cell horizontally and the height of one grid cell
vertically.
Here are two examples:
@media handheld and (grid) and (max-width: 15em) { ... }@media handheld and (grid) and (device-max-height: 7em) { ... }
7. Units
[normative]
The pixel unit used in Media Queries are of the "logical" kind (which is
reported to the user agent) rather than the "physical" kind (which often
remains unknown to the user agent). In any case, the user agent must use the
same definition of a pixel in Media Queries as is used to interpret style
sheets.
Relative units in Media Queries are based on the initial values of the
document's root element.
The "dpi" and "dpcm" units describe the resolution of an output device,
i.e. the density of the pixels. in "dots per inch" and "dots per centimeter",
respectively. These units are only used in the "resolution" media feature.
8. Acknowledgements
[informative]
This specification is the product of the W3C Working Group on Cascading
Style Sheets.
9. Appendix A.
Normative references
[CSS2]
Bert Bos; Hkon Wium Lie; Chris Lilley; Ian Jacobs. Cascading Style
Sheets, level 2. 1998. W3C Recommendation. URL: http://www.w3.org/TR/REC-CSS2
G. Klyne; F. Reynolds; C. Woodrow; H. Ohto (eds.). Composite
Capability/Preference Profiles (CC/PP): Structure and Vocabularies. 15
March 2001. W3C working draft. (Work in progress.) URL: http://www.w3.org/TR/2001/WD-CCPP-struct-vocab-20010315
L. Masinter; D. Wing; A. Mutz; K. Holtman. Media Features for
Display, Print, and Fax. March 1999. Internet RFC 2534. URL: http://www.ietf.org/rfc/rfc2534